Ink-jet printing is a non-contact reproduction technique that includes the acceptance of a digital signal representing an image and subsequent reproduction of the image onto a substrate by deposition of ink drops. Ink-jet devices typically include a printer head having one or more nozzles, each of which utilizes a static pressure ink reservoir, a small diameter orifice exiting the ink reservoir, and a voltage-gated orifice exiting the nozzle. The printer head is positioned using a two-dimensional translation mechanism. The volume of liquid dispensed in a drop is generally determined by the characteristics of the voltage-gated nozzle, while the lateral resolution of the device is usually determined by an encoder that senses the relative position of the nozzle and is controlled from a computer by a printer driver. It has been proposed to use ink-jet technology to deposit arrays of DNA-type polymers and proteins. Unfortunately, however, ink-jet printing applications have thus far been limited to non-viable biological materials. One reason for this limitation is that the high shear stresses (up to about 10 meters per second) and/or the high temperatures (up to about 300° C.) associated with ink-jet printing are believed to damage or kill viable cells. In addition, the outlet nozzles of the printer heads of most standard printers are too small to accommodate many viable cell sizes. Further, the feed mechanism of most commercially available ink-jet printers supplies paper in a circuitous route. Such circuitous systems are not always feasible when printing multiple layers or when printing onto a large, non-pliable substrate, such as a tray.
Instead, the deposition of viable cells is typically performed using robotic spotting systems, such as the Genetix Qbot system and the Beckman Coulter ORCA robot system. These systems are quite expensive and fairly slow, employing a multi-pin contact process that transfers bacteria onto a membrane filter substrate in ordered arrays with a gridding head of usually either 96 or 384 pins, which can grid up to about 3,456 colonies onto a single 8×12 centimeter substrate. Each pin is dipped into a corresponding well source plate where it picks up a target bacteria that is then directed onto the substrate. Washing and sterilization of the pins must be performed before the gridding head can be moved to new well source plates.
Techniques of forming arrays of viable cells become even more prohibitively expensive and slow when assembling cells in complex arrangements and/or when assembling more than one cell type. For example, when creating cell patterns of more than one cell type, current methods employ a two-step approach that uses complex masks to pattern cell-adhesive substrates via self-assembled monolayers or layers of certain proteins, followed by exposure of the desired cell type to the layers. In addition, such methods, while they can be utilized to form cellular arrays of more than one cell type, are still limited to fairly simple geometric patterns of cells.
As such, a need currently exists for a relatively inexpensive, quick, and efficient method of depositing arrays of viable cells onto a substrate.